Why is the camera in the viewport smaller than the focal length suggests it should be?
The size of the camera in the viewport is not the same as the focal length. Scaling the camera object won't change the focal length or the rendered image, but will change the size in viewport.

For focal length = 32mm, the camera object will look like the principal plane is 1 BlenderUnit away from the camera center. This is probably an intuitive scale so that the camera object does have a fitting size for a usual scene. If it were acutally 35 BlenderUnits big, everyone would scale it down right away.